High-frequency discharge plasma generator utilizing an auxiliary flame to start, maintain and stop the main flame



Oct. 18, 1966' MASAO SUGAWARA ETAL 3, ,36

HIGH-FREQUENCY DISCHARGE PLASMA GENERATOR UTILIZING AN AUXILIARY FLAME To START, MAINTAIN AND STOP THE MAIN FLAME 2 Sheets-Sheet 1 Filed March 5, 1963 PRIOR ART 1966' MASAO SUGAWARA ETAL 3,230,354

HIGH-FREQUENCY DISCHARGE PLASMA GENERATOR UTILIZING AN AUXILIARY FLAME TO START, MAINTAIN AND STOP THE MAIN FLAME 2 SheetsSheet 2 Filed March 5, 1963 HI GH- FREQUENCY POWER SOURCE United States Patent 3,280,364 HIGH-FREQUENCY DISCHARGE PLASMA GEN- ERA'IUR UTILIZING AN AUXILIARY FL 10 srmr, MAINTAIN AND T9]? THE MAIN FLAME Masao Sugawara, Daito-lru, Tokyo-to, Manahu Yamamoto, Ottawara-shi, Yuirio Takei, Kodaira-shi, and Seiichi Murayama, Kitatama-gun, Tokyo-to, Japan, assignors to Kabnshiiri Kaisha Hitachi Seisakusho, Tokyoto, Japan, a joint-stock company Filed Mar. 5, 1963, Ser. No. 262,914 3 (Claims. '(Cl. 315-111) This invention relates to a new high-temperature plasma generator in which high-frequency, electrodeless discharge is utilized.

As high temperature generators in which electrical discharge is used for cutting, welding, and spraying of metals, for example excitation in spectroscopic analysis, and for other purposes, generators of the type wherein a directcurrent are discharge plasma is utilized have been generally used heretofore. Such generations, however, have had several serious disadvantages as will be described in detail hereinafter.

The present invention, in its broader aspects, contemplates the elimination of such disadvantages.

The nature, principles, objects, and details of the invention will be best understood by reference to the following description, taken in conjunction with the accompanying drawings in which like parts are designated by like reference numerals, and in which:

FIG. 1 is a schematic diagram, in vertical section, showing one example of a high-temperature generator of conventional type in which a direct-current arc discharge plasma is used;

FIG. 2 is a schematic diagram, in vertical section, showing one preferred embodiment of the high-frequency discharge plasma generator according to the invention;

FIGS. 3(a) and (b) are, respectively, a plan view and a side elevational view, partly in section, showing another embodiment of the invention; and

FIG. 4 is aside elevational view, partly in section, showing still another embodiment of the invention.

Referring to FIG. 1, a typical high-temperature generator of conventional type wherein a D.C. arc discharge plasma is used is so arranged that D.C. voltage is impressed, by means of a DC power source 4, between a discharge cathode 2 disposed within a discharge container 1 and an anode 3 having an ejection orifice for discharge plasms, whereby a DC. are discharge is established within the discharge container. By introducing a gas for discharge into the said discharge container 1 through a gas inlet 6, it is possible to produce a high-temperature discharge plasma 5 extending outwardly through the aforesaid ejection orifice.

However, in the case of arc discharge by a DC. voltage as described above, the discharge electrodes, especially the anode 3 forming the ejection orifice for the hightemperature plasma, are subjected to severe erosive action, which gives rise to serious disadvantages such as the loss of stability of the discharge plasma. This erosion of the electrodes is particularly severe when this type of hightemperature generator is used for powder spraying, as a sample excitation source in spectroscopic analysis, or for some other purpose requiring the introduction of a chemically active substance into the high-temperature plasma. In such a case, the generated discharge plasma is rendered unstable until, inevitably, the generation of the required discharge plasma becomes impossible.

It is an object of the present invention to provide a new and original high-temperature generator in which a discharge which does not severely erode the discharge 13,28,364 Patented Get. 18, 1966 electrodes is utilized as the main discharge as, for example, in the case of arc discharge generated by DC. voltage as described above or by low-frequency A.C. voltage, that is, a discharge which is so-called high-frequency, electrodeless discharge.

It is another object of the invention to provide a hightemperature generator as stated above, wherein the establishment of the high-frequency, electrodeless discharge, used as the main discharge, through the use of low power is made possible, and, after establishment, the stable maintenance of this high-frequency, electrodeless discharge is made possible.

It is a further object to provide a high-temperature generator of high performance and easy operation, wherein the operational state of the discharge plasma can be readily controlled.

The foregoing objects, as well as other objects and advantages, as will presently become apparent, have been achieved by the present invention, which, briefly described, provides a high-temperature generator wherein a highfrequency circuit serving as a power source and a discharge plasma which is the load of the said circuit, are mutually separated by a means such as an insulator interposed therebetween, whereby, during discharge through the use of high-frequency electric power, it is possible to prevent torch discharge which readily occurs when such an insulator is not used, and, moreover, it is possible to accomplish the aforementioned electrodeless discharge by which there is no erosion of the electrodes.

In general, in order to cause the generation of the aforementioned electrodeless discharge in the air of atmospheric pressure, a suitable starting means is necessary. Moreover, in order to maintain the discharge, a large-size, high-power generator having an output of at least approximately kw. is necessary even when the conditions of maintaining discharge are favorable.

In the generator of the present invention, however, a high-frequency torch discharge which can be generated with low electric power is used as an auxiliary discharge for only the double function of starting and maintaining the abovesaid high-frequency electrodeless discharge. Accordingly, with even a high-frequency power of the order of 10 to 20 kw., the discharge can be started and maintained in an extremely stable manner. Furthermore, when the abovesaid high-frequency torch discharge is used as an auxiliary discharge, only a very low power sufiices for the high-frequency power required for this discharge. Moreover, by introducing a stream of inactive gas at a low flow rate to protect the torch discharge electrodes, it is possible to reduce the erosion of these electrodes to a great extent. Of course, even without the protection of the electrodes by means of the stream of inactive gas, there is almost no erosion of the electrodes, in comparison with that in the case of D.C. arc discharge, because of the low power. When the high-frequency power supplied for the high-frequency electrodeless discharge, which is the main discharge, becomes of the order of 100 kw, of course, it becomes possible to maintain this highfrequency electrodeless discharge under its own power, once it has been started. Accordingly, in this case, the abovesaid auxiliary torch discharge need function only at the time of starting. Consequently, the erosion of the electrodes for this auxiliary discharge is further reduced.

If the power to be supplied for the high-frequency electrodeless discharge is low, and this discharge cannot be independently maintained, it is possible by the arrangement of the generator of this invention to generate simultaneously the auxiliary torch discharge also, and, by connecting and disconnecting the power to be supplied for this auxiliary torch discharge, it is conveniently possible to start, maintain, or stop at will the high-frequcncy electrodeless discharge which is the main discharge.

In order to indicate still more fully the nature and details of the present invention, the following description with respect to preferred embodiment of the invention is set forth, it being understood that these embodiments are presented as illustrative only, and that they are not intended to limit the scope of the invention.

Referring to FIG. 2, which shows one embodiment of the high-frequency discharge plasma generator according to the invention wherein torch discharge is used as auxiliary discharge, high-frequency power is supplied by a high-frequency power source 7. The greater part of the output power of this power source 7 is supplied through a matching section consisting of a coaxial tube 8 and variable shorting plunger devices 9 and 10 and by means of a coupling loop 11 to a cavity resonator 12. Another part of the aforesaid output power is transmitted into a coaxial tube 15 by coupling apertures 13 and 14, which are mutually separated by a distance equal of the wavelength, and passes by way of variable shortening plunger devices 16 and 17 and by means of a coupling aperture 18 to a coaxial tube 19. The other end of the coaxial tube 15 is provided with a matched end section 20 adapted to absorb reflected waves.

At the same time of starting, a high voltage of pulse from form is impressed by a ignition pulse generator 22 between the inner conductor and outer conductor of the coaxial tube 19 as an inactive gas is introduced at a low rate through a gas inlet 21 into the coaxial tube 19, whereupon a torch discharge 23 for auxiliary use is ignited at the tip of the inner conductor 2 which protrudes into the interior of a discharge container 1 formed by a tube made of refractory, insulating material such as, for example, a quartz tube. Since the high frequency power supplied for this auxiliary torch discharge 23 is of very low value, of the order of approximately A of the total discharge power including that for the main discharge, the accompanying erosion of the electrode (conductor) 2 and other parts is extremely slight. The generation of this auxiliary torch discharge is followed by the generation of a high-frequency electrodeless discharge 24 which is the main discharge.

This high-frequency electrodeless discharge 24 is considerably more intense than the torch discharge existing by itself. Moreover, since this discharge 24 is an ele trodeless discharge, the possibility of erosion of discharge electrodes and other parts, as in the case of conventional generators depending on DC. are discharge, is completely eliminated. Into the discharge container 1 in which the discharge described above is being maintained, a suitable gas for discharge is introduced through gas inlet pipes 6, whereupon the said gas is ionized by the high-frequency electrodeless discharge, and the ionized gas so produced is ejected through an insulated nozzle 3, independently of the electric power source, to form a high-temperature plasma 25.

The matching of the cavity resonator 12 and the coaxial tube 8 is obtained by adjusting the variable shorting plunger devices 9 and and the matching of the coaxial tube 19 and the coaxial tube is obtained by adjusting the variable shorting plunger devices 16 and 17. Furthermore, since the main discharge cannot be maintained under its own power when the power for the highfrequency electrodeless discharge is low, it is possible to stop the main discharge 24 by intentionally destroying the matching between the coaxial tubes 15 and 19, by varying the variable shorting plunger devices 16 and 17, so as to extinguish the auxiliary torch discharge 23. Accordingly, the starting and stopping operations of this generator are extremely simple.

It has been found that, whereas the discharge plasma obtained by means of the generator of this invention as described above is at a temperature of the order of 6,000 degrees K., at the most, in the same when only the auxiliary torch discharge is being generated, a plasma temperature of approximately 10,000 degrees K. is obtained in the case when a high-frequency electrodeless discharge produced by an input of the order of 1 kw. is generated, and a high plasma temperature of approximately 20,000 degree K. is indicated in the case when a high-frequency electrodeless discharge produced by an input of the order of 5 kw. is generated. In this manner, there is almost no erosion of the electrodes in the generator of the present invention. Accordingly, a stable, high-temperature plasma is obtainable by the use of this generator, in which, moreover, it is possible to elevate the temperature so obtained by increasing the electric power supplied to the highfrequency, electrodeless discharge plasma.

In addition to the above-described embodiment, various modifications thereof can be resorted to. For example, in the above embodiment, instead of utilizing an electric field type of discharge through the use of the cavity resonator 12, a magnetic field type of discharge can also be utilized through the use of a high-frequency coil.

Another embodiment of the generator according to the present invention is shown in FIG. 3, in which view (a) is a plan view, and view (b) is a side elevational view, partly showing sections taken along the lines AA and B-B shown in view (a). This generator comprises, essentially, a high-frequency power source 7, a rectangular waveguide 8 for supplying high-frequency power which is joined at one end to the said power source 7, a cavity resonator 12 for generating the main discharge which is secured to the side of the said waveguide 8, and a refractory, insulating tube 1, such as a quartz tube, inserted through the center of the cavity resonator 12. A coupling slit 28 is provided between the outer wall of the cavity resonator 12 and the side wall of the rectangular waveguide 8.

At the end of the waveguide 8 opposite the end of the high-frequency power source 7, there is provided a variable shorting plunger 29. One end of a coaxial waveguide tube 15 is coupled by way of a coupler 30 to the rectangular waveguide 8, and the other end is connected to the lower end of the insulating tube I inserted through the center of the cavity resonator 12. The insulating tube 1 is provided with a torch discharge electrode 2 formed by extending the conductor within the coaxial waveguide tube 15 into the interior of the insulating tube 1. The insulating tube 1 is further provided with an ignition conductor 31 which is passed through and supported by the wall of the tube 1. The insulating tube 1 is provided with a gas inlet pipe 6 connected thereto. At the end of the aforesaid coupler 30 opposite the end of the coaxial waveguide tube 15, there is provided a tuning plunger 9.

The above-described constructional arrangement is so adapted that the distance between the coupler 30 and the center of aforementioned coupling slit 28 of the re"- tangular waveguide 8 and the cavity resonator 12 is equal to A; of the guide wavelength or to an odd-number multiple of A1 of the guide wavelength.

The high-frequency discharge plasma generator of the above construction operates as follows. A gas is caused to flow at a suitable flow rate through the gas inlet pipe 6 into the insulating tube 1. At the same time, highfrequency power is supplied into the rectangular waveguide 8. Then, by adjusting the variable shorting plungers 29 and 9, an intense high-frequency electric field is created at the extreme end of the coaxial waveguide tube '15, that is, the end where the torch discharge electrode 2 is extended, after which the inner end of the ignition conductor 31 is caused to approach the electrode 2, whereupon a torch discharge 23 is created at the electrode 2. As a result, the main discharge 24 is induced by this torch discharge 23 and is created in the insulating tube I inserted through the center of the cavity resonator 12. In this case, however, since the inner wall surface of the cavity resonator 12 is insulated by the insulating tube 1, there is no possibility,

similarly as in the case of electr-odeless discharge, of the wall surface being eroded.

When, with the generator in the above state, the variable shorting plunger 29 is adjusted and set at a position which is at a distance of /z or an integral multiple thereof of the guide wavelength from the coupler 30, measured from the top of electrode 2 to the bottom of waveguide 8, the portion of the coupler 30 is placed in a shorted state, and

'the power supply to the coaxial waveguide tube is cut off. As a result, the aforesaid torch discharge is extinguished, and all of the high-frequency power supplied to the rectangular waveguide 8 is supplied through the coupling slit 28 into the main discharge space of the cavity resonator 12, whereby the main discharge is continuously maintained. The temperature of this high-frequency electrodeless discharge plasma, which is the main discharge, reaches an extremely high value and ionizes the gas introduced through the inlet pipe 6, whereby a hightemperature plasma is ejected.

Furthermore, if the variable shorting plunger 29 is adjusted and set at a suitable position which is different from the aforesaid position at a distance of /2 or a wholenumber multiple thereof of the guide wavelength from the coupler 30, the power supplied to the rectangular waveguide 8 will be divided into suitable proportions to the cavity resonator 12 and the coaxial waveguide tube 1-5, and, since the torch discharge will then be maintained together with the main discharge, which will then be fostered by the torch discharge, there will be no possibility of the main discharge being extinguished, even if the power supplied from the power source should decrease.

While, in the above-described embodiment, a variable shorting plunger 29 is provided at one end of the rectangular waveguide 8, and by adjusting this variable shorting plunger 29, the starting and extinguishing of the torch discharge as well as the control of the main discharge such as its induction and maintenance are accomplished, these operations can be completely controlled in an automatic manner through the use of a plasma generator such as that of the following embodiment of the invention shown in FIG. 4.

In the plasma generator shown in FIG. 4, the end of the rectangular waveguide 8 opposite the power source end is closed by a fixed shorting plate 32, which is so formed and positioned as to be at a distance of A of the guide wavelength from the coupler 30. At the same time, the coaxial waveguide 15 is so formed that its axial length is /2 of the wavelength or a whole number multiple thereof.

When this generator is in the state wherein discharge is not being carried out at the torch discharge electrode 2, since the end of the coaxial waveguide tube 15 near this electrode 2 can then be regarded :as being in an open state, the impedance as viewed from the coupler 30 toward the coaxial waveguide tube 15 is very high, and an intense high-frequency electric field is induced on the side of the torch discharge electrode 2. Then, when the ignition conductor 31 is caused to approach the electrode 2 to create the torch discharge, the main discharge is induced and created within the discharge space in the center of the cavity resonator 12. However, since the generation of the torch discharge causes this end of the coaxial waveguide tube 15 to be in an almost short-circuited state, the impedance of the coupler 30 part is also decreased to a small value. Accordingly, the impedance as viewed from the coupling slit 28 toward the side of the shorting plate 32 becomes very large, and the influence of the coaxial waveguide tube 15 becomes very small. As a result, the greater part of the power is supplied to the cavity resonator 12 to maintain the main discharge.

That is, in the above-described embodiment, it becomes possible to carry out, in a completely automatic manner, switching between the torch discharge and the main discharge at the time of starting, at the time of stopping, or during maintenance of discharge.

As will be apparent from the foregoing description, the present invention has the advantageous feature, during its carrying out of electrodeless discharge, of not requiring a separate power source for each of the main discharge circuit and the torch discharge circuit for starting, using commonly the same high-frequency power source, thereby being capable of switching and supplying the resulting power to the torch discharge circuit and the main discharge circuit, whereby the induction and maintenance of the main discharge can be carried out readily and positively.

The generator of the present invention having the above-described construction, has advantages such as that of being, in comparison with conventional high-temperat-ure generators, capable of generating discharge plasmas of much higher temperatures and of accomplishing starting, maintaining, and stopping discharge in an easy and, moreover, stable manner, and, in addition incurring extremely little erosion of the electrodes. Furthermore, the generator of the present invention is effectively applicable for use in cases such as, through the use of the hightemperature discharge plasmas so produced, carrying out chemical reaction (synthesis and dissociation) at ultra high temperatures or carrying out working processes on materials of high melting point.

Although the present invention has been described in conjunction with particular embodiments thereof, it is to be be understood that modifications and variations may be resorted to therein without departing from the spirit and scope of the invention, as those skilled in the art will readily understood, and such modifications and variations are to be considered to be within the purview and scope of the invention and appended claims.

What is claimed is:

1. A high-frequency discharge plasma generator comprising a discharge region, and a single high-frequency electrical power supply capable of causing the creation of a high-frequency electrodeless discharge within the said discharge region, and imultaneously causing the creation within the said discharge region of an auxiliary torch discharge which accomplish starting, maintenance, or stopping of the said high-frequency electrodeless discharge.

2. A high-frequency discharge plasma generator of the type comprising a discharge region, a high-frequency electrical power supply causing the creation of a highfrequency electrodeless discharge within the said discharge region, and means for causing the creation within the said discharge region of an auxiliary torch discharge which accomplishes starting, maintenance, or stopping of the said high-frequency electrodcless discharge, wherein the said high-frequency electrical power supply means consists of a power source and a waveguide; a cavity resonator forming an electrodeless main discharge circuit and enclosing the said discharge region is coupled to the said waveguide; and the means for causing the creation of an auxiliary torch discharge comprises a coaxial waveguide tube, an auxiliary torch discharge circuit coupled by way of the said coaxial waveguide tube to the said waveguide, and shorting mean provided at one end of the said waveguide and adapted to control, adjustably, the switching and supplying of high-frequency electrical powor to the aforesaid two discharge circuits.

3. A high-frequency discharge plasma generator of the type comprising a discharge region, high-frequency electrical power supply means capable of causing the creation of a high-frequency electrodeless discharge within the said discharge region, and means for causing the creation within the said discharge region of an auxiliary torch discharge which accomplishes starting, maintenance, or stopping of the said high-frequency electrodeless discharge, wherein: the said high-frequency electrical power supply means consists of a power source and a waveguide; a cavity resonator forming an electrodeless main discharge circuit enclosing the said discharge region coupled to the said waveguide; and the said means for caus- 7 ing the creation of an auxiliary torch discharge comprises a coaxial waveguide tube formed to have an axial length which is equal to /2 of the guide wavelength or a wholenurnber multiple of /2 of the guide wavelength, measured from the tip of said torch discharge to the bottom of said waveguide, an auxiliary torch discharge circuit coupled by way of the said coaxial waveguide tube to the said waveguide, and means for automatically varying the impedance of the coupling point of the said coaxial waveguide tube and the said waveguide in accordance with the ignition and extinguishing of the said auxiliary torch discharge so as to control automatically the supply of electrical power to the said main discharge circuit.

References Cited by the Examiner UNITED STATES PATENTS 11/1951 Volff et al. 315l66 9/1964 Sleeper 33313 

1. A HIGH-FREQUENCY DISCHARGE PLASMA GENERATOR COMPRISING A DISCHARGE REGION, AND A SINGLE HIGH-FREQUENCY ELECTRICAL POWER SUPPLY CAPABLE OF CAUSING THE CREATION OF A HIGH-FREQUENCY ELECTRODELESS DISCHARGE WITHIN THE SAID DISCHARGE REGION, AND SIMULTANEOUSLY CAUSING THE CREATION WITHIN THE SAID DISCHARGE REGION OF AN AUXILIARY TORCH DISCHARGE WHICH ACCOMPLISH STARTING, MAINTENANCE, OR STOPPING OF THE SAID HIGH-FREQUENCY ELECTRODELESS DISCHARGE. 